U.S. Pat. No. 4,429,990 issued Feb. 7, 1984 to E. J. Tamary shows a pressure roller fuser of a type presently commercially used to fix toner images to support sheets. An important aspect of that disclosure is a mechanism for applying release liquid to a fusing roller which contacts the toner image. That liquid applying mechanism, commonly called a rotating wick, includes a hollow porous roller, which is supplied with fusing oil internally. The applicator has an inner supply tube with holes in it and is covered by a porous material having a surface of wool or a heat resistant synthetic wicking material. The applicator is rotatable by the fusing roller. Because the applicator rotates with the roller, the wick thoroughly applies the liquid with a minimum of wear to the roller. The applicator is movable into and out of engagement with the roller according to a program which prevents excess buildup of oil on the roller which otherwise would stain the image bearing sheet. This general type of structure is relatively inexpensive to manufacture and is effective in applying oil to rollers, both of the type in which oil is applied to one roller and also the type shown in the patent where the oil is supplied by separate wicks to both rollers.
However, because the wick structure directly contacts the roller and rolls with it, it has a tendency to leave the liquid in a fine pattern consisting of spots of locally excessive liquid. These spots may impede transfer of heat energy from the fusing roller to the toner. With ordinary paper support sheets, the release liquid is quickly absorbed and the spots of excess liquid have no adverse effect. Resin based stock, for example transparency stock, does not absorb the liquid and small spots of incomplete fusing can result. These spots are not generally noticed when black toner is used on transparencies. However, even a small amount of incomplete fusing of a color transparency will show up as gray or black spots in a projected color image using that transparency. The present tendency of the art is to use very viscous silicone oils as the release liquid has increased this problem of local excessive liquid.
A number of references show approaches to improving fusing oil control and fusing transparency; see for example U.S. Pat. Nos. 4,549,803 and 4,593,992. In both these patents fixing conditions are changed when the sheet carrying the toner image is synthetic resin rather than paper. To improve fixing for transparency stock, an obvious solution is to slow the fixing apparatus down or increase the temperature to thereby apply more fusing power to each unit of area to be fused. Obviously, if transparency stock absorbs less release liquid, the amount of liquid applied must be reduced as well. In these two patents this is accomplished by articulating a wick that applies release liquid to an application roller or by control of various spreading and cleaning devices in the system. These patents deal only with the problem of excess liquid on transparencies in general not with the problem of locally excessive spots of liquid imparted by the wicking structure itself.
The Tamary patent is representative of a number of other patents which show articulating wicks for pressure roller fixers; see for example U.S. Pat. Nos. 4,008,955; 4,045,165 and 4,272,666.
Commonly-assigned U.S. Patent Application Ser. No. 223,829 to Mills et al entitled "Fixing Method And Apparatus," filed July 25, 1988 discloses a solution to this latter problem of locally excessive spots of liquid imparted by the wicking structure as well as the problem of generally excess liquid on transparency stock. According to that application, the rotatable wick is moved out of engagement with the roller surface a sufficient time prior to contact of the transparency with that surface to give surface tension on the liquid an opportunity to eliminate locally irregular excesses of liquid. In addition, a sheet of paper stock can be run through the fuser to both spread and absorb liquid prior to fusing a transparency.
This method has been shown to be extremely effective in both reducing the total amount of liquid applied to the transparency and in removing local excesses of liquid. However, despite its quality advantages it inherently slows the fusing process by increasing the necessary time between sheets.